I was poking around some other SSRS blogs from folks here on the product team, and I found this post from Lukasz about troubleshooting Reporting Services . It is a great overview of some of the guidance we have given in the form of whitepapers, KB artciles...

Reporting Services uses a temporary database for storage of objects which are, well, temporary . For example, report snapshots which are associated with a particular user session as the result of a live execution. Cached report snapshots are also stored...